The present invention relates generally to devices and methods for retreading tires, and more particularly to devices and methods for buffing a tire casing to remove worn tread.
In general, devices for removing the tread of worn tires, often called buffers or raspers, are well known. Tire buffing is part of the tire retreading operation. The method of tire retreading described herein is commonly referred to as xe2x80x9ccold-process retreading.xe2x80x9d
Typically the tire casing selected for retreading is buffed to remove excess rubber to provide a substantially evenly-textured crown for receiving a pre-cured tread strip and to provide a predetermined tire casing profile. Tire casings usually include a belt package (a package of steel belts or cables) underlying the road-engaging surface (e.g., the original tread) of the tire. Prior to retreading, the casing must be buffed, generally to a predetermined characteristic crown radius corresponding to the upper contour of the belt package. The casing is buffed to leave only a predetermined thickness, e.g., {fraction (3/32)} of an inch, of material remaining over the top belt. The shoulder of the casing is also buffed (trimmed) to eliminate or reduce voids or patterns in the shoulder created by the original tread, and to provide, typically, a relatively straight profile between the casing side walls and the crown. A worn casing from each of the various models and sizes of new tires has a characteristic tire casing profile of a particular crown width, crown buffing radius and shoulder trim angle which must be created as an initial step in the buffing process.
After being buffed, the tire casing may then be examined for injuries, which are skived and filled with a repair gum. After completion of the skiving process, the buffed surface may be sprayed with tire cement that provides a tacky surface for application of a suitable layer of bonding material, such as cushion gum. Conventionally, the cushion gum is a layer of uncured rubber material, which optionally includes a low temperature vulcanizing agent and accelerator. The cushion gum can be placed over the crown. In some retreading operations, the spray cement can be omitted.
Then a cured tread strip, typically of a width corresponding to the width of the crown of the casing is cut to the length corresponding to the casing circumference and is disposed over the casing crown. Alternatively, continuous replacement treads in the shape of a ring (i.e., ring treads) have also been used to retread the buffed casing. A roller pressing process, commonly referred to as stitching, is next performed on the assembly to force air from between the tread strip and casing.
After stitching the tire assembly, which comprises the tire casing, the cushion gum and the tread, the assembly can be placed within a flexible rubber envelope. An airtight seal can be created between the envelope and the bead of the tire casing. The entire envelope, with the tire assembly disposed therein, can be placed within a curing chamber and subjected to elevated pressure and temperature for a predetermined period of time. The combination of exposure to elevated pressure and temperature for a duration of time binds the cushion gum to both the tire casing and the new tire tread.
The tire buffing step of the retreading process can require the face of the casing, which receives the replacement tire tread, to be buffed to a predetermined crown radius within a relatively small tolerance. In addition, with the advent of new replacement tire tread designs, such as the contoured replacement tread disclosed in U.S. Pat. No. 5,277,727, issued Jan. 11, 1994, it can also be necessary to ensure that the shoulder radius is also buffed precisely to a predetermined arc, or radius.
In present retreading processes, it is important that the surface of the tire casing be carefully buffed about the shoulder areas of the tire to ensure that the tread layer width is approximately the same as the buffed surface of the casing. If the shoulder areas are not sufficiently buffed and trimmed, the tread edges may come loose and/or the cushion gum extending beyond the tread edges will not bond to the casing shoulder. Such problems can reduce the longevity of the retreaded tire and adversely impact the appearance of the retreaded tire.
The tire casing is buffed to remove tread material and to achieve a desired surface texture. Typically, it is desired for the crown and the shoulders of the tire casing to have different textures. The surface texture can be measured on a visual scale propounded by the Rubber Manufacturer""s Association (RMA) with a numerical value between 1 and 6 with 1 being the finest and 6 being the roughest texture. Typically, it is desired for the crown of the tire casing to have a RMA number of between 3 and 4 and for the shoulder to have an RMA number of about 2. The shoulders of the tire casing typically are subjected to the largest stresses encountered by the tire during service. The finer texture on the shoulders promotes better adhesion of the tire tread to the casing at these high stresses. If the shoulders received too much texturing, crack propagation in these areas is more likely. In instances where the shoulder receives too little texturing, the bond between the tire casing and the tread is poorer.
The current state of the art in tire casing buffing allows for a multiplicity of tread removal passes, substantially in a sideways, pass after pass method. Known manual devices which require an operator to physically direct the buffing machine""s removal direction and speed produce time periods between tread removal passes where the rate of tread rubber removal is less than optimum. Known automated devices in which the buffing machine""s removal direction and speed is predetermined by an operator also produce time periods of less than optimum tread rubber removal.
Thus, there exists a need for a tire buffing machine which is easy to use and which improves tire buffing efficiency.
The present invention provides a tire buffing apparatus for buffing a tire casing as part of a retread operation. The tire buffer can include a rasp pedestal having a rasp head and a texturing device. A moving assembly can be connected to the rasp pedestal for selectively moving the rasp pedestal along a pair of perpendicular axes. The rasp pedestal can rotate about a vertical axis. A tire hub assembly can be included for rotating the tire casing. The tire buffer can be operated at an operator station by interfacing with a control unit. The tire buffer can automatically buff a tire casing to achieve a predetermined tire casing profile and impart a texture thereto. The tire buffer can include a tire location mechanism and a tire measurement mechanism mounted to the rasp pedestal.
The present invention can improve the overall efficiency of tread rubber removal during the buffing step of a tire retreading process. The control unit can monitor an operating parameter of the rasp pedestal or the tire hub assembly during the buffing sequence, such as the current draw of the rasp drive motor, for example, compare the actual value of the operating parameter to a predetermined target value of the parameter, and adjust an operating characteristic of the rasp pedestal, the tire hub assembly, or both, such as the traverse speed of the rasp pedestal with respect to the tire casing as it makes a pass, for example, such that the actual value of the operating parameter is urged toward the calculated target value of the operating parameter.
In one aspect of the invention, the main tread cutter of the tire buffer can be configured such that the cutter speed, i.e., the traverse speed of the cutter across the width of the tire casing, and the cutter feed, i.e., the depth of cut of the tread cutter, can both be adjustable. In one embodiment, an operator can select a feed rate for the cutter before the buffing operation begins. During the buffing cycle, the tire buffer can monitor the current draw on the motor operating the cutter and compare the actual current draw against a predetermined desired current draw. The cutter speed can be varied in response to any measured difference between the actual current draw and the predetermined current draw, with the speed increasing to increase the actual current draw and with the speed decreasing to decrease the actual current draw.
In a tire measurement operation, the rasp head can be moved toward the tire casing with the blades of the rasp being stationary and the tire casing, mounted to the tire chuck of the tire hub assembly, rotating. The rasp can be moved with respect to the tire hub assembly via the moving assembly. The rasp can be moved with respect to the tire hub assembly such that the rasp contacts the tire casing. Upon contacting the tire casing, the blades of the rasp head rotate in response to the rotation of the tire casing. A sensor can sense when the rasp blades start spinning and signal such occurrence to a control unit, thereby indicating the size of the tire casing.
The tire buffer includes an automated buffing operation. The main rasp, upon completion of the buffing step, can be positioned with respect to the tire casing to trim the shoulders thereof. The desired shoulder geometry can vary depending on the tire tread being used in the retreading process.
In another aspect of the invention, the tire buffer includes an automated shoulder texturing feature carried out by a texturing device, such as a wire brush, for example, mounted on the same spindle as the main tread cutter, the rasp head. By providing a tire buffer which has automated buffing and shoulder texturing features, the output of buffed tire casings from said machine is improved by facilitating the buffing process to deliver a more consistent texture profile across the tire casing and from tire casing to tire casing.
In the automated shoulder texturing operation, the texturing device is moved with respect to the tire casing with the texturing device being aligned with one of the shoulders. The texturing device is moved toward the tire casing with the texturing device rotating. A control unit monitors the current required to operate the texturing device to gauge the degree of contacting force between the shoulder and the texturing device. The texturing device is moved into the tire casing until the motor current reaches a predetermined level, at which point the texturing device stops moving closer to the tire casing. The texturing device is operated at this position for a predetermined amount of time to provide the desired shoulder texture. The rasp pedestal is moved with respect to the tire casing such that the texturing device is aligned with the other shoulder, and the same process is repeated for that shoulder.
The features of the present invention will become apparent to one of ordinary skill in the art upon reading the detailed description, in conjunction with the accompanying drawings, provided herein.